Method for reducing the cross section of semiconductor rods by molten-zone stretching

ABSTRACT

METHOD OF FLOATING ZONE MELTING A VERTICALLY MOUNTED ROD AND SIMULTANEOUSLY STRETCHING THE MOLTEN ZONE FOR REDUCING THE ROD IN CROSS SECTION INCLUDES PLACING AROUND THE TOP OF A THIN CRYSTAL SEED AN INDUCTION HEATER COIL HAVING TURNS OF A DIAMETER LARGER THAN THE THICKNESS OF THE THIN CRYSTAL SEED, FORMING THE LOWER END OF A THICK ROD HAVING A DIAMETER GREATER THAN 25 MM. A GREATER THAN THE DIAMETER OF A PLURALITY OF INNER TURNS OF THE COIL BUT SMALLER THAN THE DIAMETER OF AT LEAST ONE OF THE OUTER TURNS OF THE COIL APPROXIMATELY INTO THE SHAPE OF THE MOLTEN ZONE TO BE PRODUCED BY THE COIL, CONTACTING THE LOWER END OF THE THICK ROD WITH THE TOP OF THE SEED AND HEATING THE JUNCTION THEREOF WTH THE COIL SO THAT THE TURNS OF THE COIL ARE POSITIONED VERTICALLY BENEATH THE THICK PORTION, AND THEREAFTER COMMENCING THE ZONE MELTING AND STRETCHING OPERATIONS.

Feb. 16, 1971 w. KELLER ETAL 3,563,810

METHOD FOR REDUCING THE CROSS SECTION OF SEMICONDUCTOR RODS BY MOLTEN-ZONE STRETCHING Original Filed July 10, 1964 Int. Cl. B613 17/10 US. Cl. 1481.6 2 Claims ABSTRACT OF THE DISCLOSURE Method of floating zone melting a vertically mounted rod and simultaneously stretching the molten zone for reducing the rod in cross section includes placing around the top of a thin crystal seed an induction heater coil having turns of a diameter larger than the thickness of the thin crystal seed, forming the lower end of a thick rod having a diameter greater than 25 mm. and greater than the diameter of a plurality of inner turns of the coil but smaller than the diameter of at least one of the outer turns of the coil approximately into the shape of the molten zone to be produced by the coil, contacting the lower end of the thick rod with the top of the seed and heating the junction thereof with the coil so that the turns of the coil are positioned verticall beneath the thick portion, and thereafter commencing the zone melting and stretching operations.

This application is a divisional of Ser. No. 757,217, filed Aug. 28, 1968, now abandoned, which is a continuation of Ser. No. 381,744, filed July 10, 1964, now abandoned.

Our invention relates to a method and means for reducing the cross section of semiconductor rods by subjecting them to floating zone melting and simultaneously stretching the molten zone by moving one end of the rod away from the other. This method and suitable apparatus for its performance are known from Pats. 3,030,194.

According to the apparatus described in the patent, a vertically mounted semiconductor rod, secured at both ends in respective holders, is surrounded by a heater which produces an axially narrow molten zone in the rod while the rod holders are being moved away from each other thus stretching the molten zone and reducing the cross section of the recrystallizing rod. As a result, for example, the ultimate rod can be sliced into semiconductor plates of the size required for the production of electronic semiconductor devices, thus eliminating the time consuming and wasteful subdivision of slices whose cross section is larger than needed.

The heating of the molten zone is effected by means of an annular strip of tungsten traversed by electrical current to assume a temperature of about 2000 C. The transfer of heat from the heater to the semiconductor rod is by radiation. The attainable maximum reduction in cross section is mm.

In cases where to be reduced in the cross section of the original rod diameter is rather large or where a greater than the above-mentioned reduction in cross section is desired, considerable difficulties have been encountered because under such conditions the molten zone is no longer sufficiently stable and can drip off since the thin portion of the rod can no longer sustain the molten zone.

United States Patent 0 3,563,810 Patented Feb. 16, 1971 It is an object of the invention to minimize these difficulties and to afford or improve the drawing-thin of rods having a considerably larger cross section than permissible in the above-mentioned known apparatus.

To this end, and in accordance with the invention, we perform the floating-zone stretching process fundamentally in the above-described manner with the rod portion of reduced cross section located vertically below the thick portion, and we employ as zone heater an induction winding and have it surround the top of the thin rod portion so closely that one or more turns of the induction coil are situated vertically beneath the thick rod portion. Preferably the induction coil has the shape of a flat spiral.

By virtue of the invention, semiconductor rods having a diameter of more than 25 mm. can readily be processed into thinner rods of monocrystalline constitution.

The drawing shows schematically in FIGS. 1, 2 and 3 respectively different embodiments of the invention by way of example.

It should be understood that the illustrated rod portions and induction heater coils form part of an apparatus which may be identical with the one illustrated in FIGS. 1 to 3 of Pat. 3,030,194 so that reference may be had to the patent for a detailed description of components not essential to the present invention proper.

According to FIG. 1 of the present disclosure, an induction heater coil 2 surrounds the bottom portion of a molten zone 3 between an upper, thick rod portion 4 and a lower, thin rod portion 5. The upper end of portion 4 is secured in a holder, and the lower end of portion 5 is likewise secured in a holder (both holders not shown) which are being moved apart during the zone-melting operation. The coil 2 is designed as a flat spiral. The inner winding turns of coil 2 are located vertically beneath the thick rod portion 4 and thus impose a levitating action upon the molten zone 3 by means of the magnetic field of the coil. The molten zone is thus sustained in its position and much better prevented from dripping off than is the case in the apparatus previously known.

For example, the thick rod 4 processed according to FIG. 1 may have a diameter of 40 mm. and the thin rod portion 5 recrystallizing from the molten zone 3 may have a diameter of 20 mm. With an inner coil diameter of about 25 mm., the levitating action of the magnetic field is ample for securing a stable zone-stretching operation.

The operation can be performed in known manner in vacuum or in a protective gas. Preferably applied is a vacuum. The travel of the molten zone relative to the semiconductor rod is produced by moving the heater coil, or by moving the rod holders with respect to the heater coil which then may remain fixed. In the latter case, since the cross section of the semiconductor rod is very greatly reduced, the two rod portions 4 and 5 must move at respectively different speeds. That is, with a fixed heater coil 2, the thick rod portion 4 is to be moved downward at relatively low speed toward the coil 2, while simultaneously the thin rod portion 5 is pulled downwardly at a correspondingly higher speed. For example, with a reduction from 40 to 20 mm. in diameter, the thin rod portion must move at four times the speed of the thick portion relative to the heater coil.

The method, of course, can be performed also by maintaining the upper rod portion 4 at rest and moving only the lower portion, in which case the heater coil 2 must also be moved.

It has been found advantageous to move the thin rod portion at highest feasible speed with respect to the heater winding 2; i.e. at a speed which, if possible, should be higher than 3 mm. per minute. For example, this speed may be 4 mm./min. or may be increased up to 6 mm./ min. Increasing the speed of the thin rod portion improves the crystal quality because the lower boundary 6 of the molten zone 3 is nearly planar under these conditions. Hence, the crystal growing out of the melt exhibits only slight crystalline disturbances at the solid-toliquid boundary because the tensions are substantially equalized.

The shape of the heater coil can be adapted to any particular requirements. Thus FIG. 2 shows a funnel-shaped heater coil 2 whose levitating efiect has likewise been found to be excellent.

We have found it advisable to approximately adapt the shape of the thicker rod portion, prior to commencing the zone melting, to the expected shape of the molten zone. Otherwise it may happen that some semiconductor material can drip off at the beginning of the zone-melting operation and may short-circuit the turns of the heater coil or cause damage in the zone-melting equipment. FIG. 3 shows the lower end of the thick rod 4 which has been given a frustoconical or somewhat pointed shape by grinding or other suitable mechanical machining. The thick rod 4 consists for example of polycrystalline silicon or other semiconductor material produced by pyrolytic precipitation from the gaseous phase. Also shown in FIG. 3 is a thin crystal seed 5a which is monocrystalline and which is to be fused to the lower end of the thick rod 4 when commencing the above-described zone-stretching operation.

We claim:

1. In the method of floating zone melting a vertically mounted rod and simultaneously stretching the molten zone for reducing the rod in cross section, the improvement which comprises placing around the top of a thin crystal seed an induction heater coil having turns of a diameter larger than the thickness of the thin crystal seed, forming the lower end of a thick rod having a diameter greater than 25 mm. and greater than the diameter of a plurality of inner turns of the coil but smaller than the diameter of at least one of the outer turns of the coil approximately into the shape of the molten zone to be produced by the coil, contacting the lower end of the thick rod with the top of the seed and heating the junction thereof with the coil so that the turns of the coil are positioned vertically beneath the thick portion, and thereafter commencing the zone melting and stretching operation.

2. Method according to claim 1, which comprises sharpening the lower end of the thick rod to a substantially conical shape before commencing the zone melting and stretching operation.

References Cited UNITED STATES PATENTS 2,981,687 4/1961 Parmee 252-623 2,992,311 7/1961 Keller 219-10.77 3,030,189 4/1962 Schweikert et a1. 232235 3,030,194 4/1962 Emeis 23301 3,113,841 12/1963 Reuschel 23223.5 3,121,619 2/1964 Scholte 23301 3,136,876 6/1964 Crosthwait 21910.77 3,232,716 2/1966 Quast et al. 23273 3,351,433 11/1967 Keller 23301 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 23273, 301 

